The present invention relates generally to the field of medical imaging. In particular, the following techniques relate to dual energy computed tomography imaging systems and basis material decomposition within such systems.
Computed tomography (CT) imaging systems measure the intensity of X-ray beams passed through a patient from numerous angles. With sufficient angular coverage around the patient, cross-sectional images can be formed revealing the inner structure of the scanned object. The images are typically displayed on a cathode ray tube, and may be printed or reproduced on film. A virtual 3-D image may also be produced by a CT examination.
CT scanners operate by projecting X-ray beams from an X-ray source through an attenuating object, such as a patient. The X-ray beams may be collimated between the source and the object into a fan or cone shape, depending of the configuration of the detector, optimal patient exposure, or other factors. The attenuated beams are then detected by a set of detector elements. The detector element produces a signal based on the intensity of the X-ray beams. The measured data are then processed to represent the line integrals of the attenuation coefficients of the object along the ray paths. The processed data are typically called projections. By using reconstruction techniques, such as filtered backprojection, cross-sectional images are formulated from the projections. Adjacent cross-sectional images may be displayed together to render a volume representing the imaged region of the object or patient.
The X-ray beam attenuation caused by a given length of a material, such as bone or soft tissue, may be represented as an attenuation coefficient for that material. The attenuation coefficient of a material is a function of two separate events that may occur when an X-ray beam passes through a given length of the material. The first events, known as Compton scatter, denotes the tendency of an X-ray photon passing through the length of the material to be scattered or diverted from the original beam path, with a resultant change in energy. The second event, known as photoelectric absorption, denotes the tendency of an X-ray photon passing through the length of the material to be absorbed by the material.
As one might expect, different materials differ in their scatter and absorption properties, resulting in different attenuation coefficients for different materials. In particular, the probability of Compton scattering depends in part on the electron density of the imaged material and the probability of photoelectric absorption depends in part on the atomic number of the imaged material, i.e., the greater the atomic number, the greater the likelihood of absorption. Furthermore, both the Compton scattering effect and photoelectric absorption depend in part on the energy of the X-ray beam. As a result, materials can be distinguished from one another based upon the relative importance of the photoelectric absorption and Compton scattering effects in X-ray attenuation by the material.
In particular, measurement of the attenuation produced by a material at two X-ray energy levels or spectra, i.e., at dual energies, may allow for respective Compton scattering and photoelectric absorption contributions to be quantified for a material at the X-ray energy levels employed. In this manner, dual energy CT may provide spatial information in conjunction with information regarding the physical density and/or effective atomic number of the material or materials within the imaging volume. Using the spatial and density and/or atomic number information, an operator may reconstruct images that predominantly display selected materials, such as bone, soft tissue, or contrast agent, which differ in their atomic number or density. In this manner, a bone image, a soft tissue image, a contrast agent image, and so forth may be reconstructed which predominantly displays the material of interest. These images may in turn be associated to form a volume rendering of the material of interest which may be useful in determining bone density or deterioration, soft tissue damage, contrast agent perfusion, and so forth.
The process of decomposing the acquired data into the data associated with the basis materials, i.e., the materials for which images are desired, may be complex. In particular, the basis material decomposition (BMD) process typically involves a complicated mathematical inversion process. The inversion process may be inadequate to account for realistic system response, particularly for complex detector configurations or imperfect detector characteristics, and thus may be unable to accurately perform BMD in some circumstances. One solution to this imprecision has been to iteratively perform the inversion process until acceptable results are obtained. This solution, however, may be time-consuming and wasteful of computer resources. A rapid and accurate technique for performing BMD is therefore desirable.